In accordance with the digitalizing trend of the image signal processing method picture-in-picture, (PIP) televisions appeared, and these are indicative of those televisions which are capable of displaying sub-pictures in the form of moving or stationary images occupying a part or whole of the main picture.
Such a PIP television could display a main picture through a tuner, off-air broadcast source, and display moving or stationary images through a video tape recorder (VTR) or other non-broadcast video sources as sub-pictures during the early stage. Thereafter, a technique of swapping the main picture and sub-pictures was developed, and also, a system capable of simultaneously receiving two off-air broadcast video sources such as a 2-tuner system was developed.
Recently, video sources are diversified beyond off-air broadcast or VTR, for example, cable borne programming such as CATV, teletext or videotex, and RCG sources in computer graphics, video disc player, and video camera, and therefore, corresponding signal characteristics are also diversified. In order to process and display such diversified signals, the digitalization of the processing of video signals has been increasingly promoted.
The digital tuner which has appeared in conformity with such trends is capable of receiving video signals from a plurality of broadcasting sources simultaneously, and is capable of displaying them after multiplexing them, so that a plurality of broadcasting programs can be simultaneously displayed on the screen. This technique applied to the PIP TV is the so-called multiple PIP (M-PIP).
Such a multiple PIP TV can display a plurality of sub-pictures within a main picture, and therefore, can show various new functions which are not seen in the preceding PIP TV. One of the the new functions is the multichannel scanning function, and according to this function, the digital signals from the digital tuner can be successively output, so that programs broadcast by different broadcasting stations or programs from different video signals sources can be scanned, thereby performing the multichannel scanning function.
The conventional multichannel scanning method of such a multiple PIP TV is shown in FIG. 1, which was adopted, for example, by Matsushita Electric Company of Japan in its Hi-Fi VTR "NV-D21" sold from Dec. 1, 1986 (refer to the journal "Video Saloon", Decemeber 1986, page 139).
In this method, first, the user manipulates the ten keys to input data for the channel to be scanned and to store the data into the system, and then, the user presses the channel scan key to set a plurality of sub-pictures (4 sub-pictures in FIG. 1) on the screen. At the same time, the system successively writes and reads, to and from the relevant memory, the video signals for the selected and memorized channel, so that the stationary images for the memorized channels should be output as respective sub-pictures, thereby performing multichannel scanning.
The controlling of sub-pictures in the conventional multichannel scanning method will be described in further detail. First, a plurality of sub-pictures are turned on to the main picture, then the image for the first channel memorized is output to the first sub-picture P1, the the position (and the field of the memory for writing and reading) of the sub-picture to be output is moved to a second sub-picture P2, the selection of the channel is shifted up or down at the same time, and then, the signals for the memorized next channel are received, written and read out, thereby displaying the second sub-picture P2. Such operations are repeated to output the images for the respective channels until the final sub-picture (P4 in FIG. 1) is reached, the sub-picture thereupon being turned off. However, in this conventional multichannel scanning method, the user has the select the channels to be scanned to store them into the system and therefore, the operation is very troublesome. Further, if the user erroneously selects a channel lacking output signals, then only noise will be generated in the sub-picture, thereby failing proper scannings. Further, in this conventional method, the simultaneously scannable number of channels is limited to number of sub-pictures, and therefore, if it is desired that the channels be scanned in number greater than the number of sub-pictures, a reselection and a rescanning have to be performed, thereby causing further trouble in the operation of the system.